Easy answer: It’s my fault. The functions that are being used for creating a backup into a .zip file run as a single process, using more and more of your server’s memory. In the next version, this will be gone, I’m planning to split the processes so that even users with small allowed memory sizes can use it properly.

What you can do:

1. Put ini_set('memory_limit', '-1'); into your functions.php and see if it works. If not, go to step 2.
2. Ask your server provider to open the php.ini of your server and increase the value for memory_limit.